Fixes for Issues 2 and 3

I've fixed issues 2 and 3, checked in as separate commits if you want to break them apart.  Specifically, I've done the following:

1) Added a file existence check to the execution of the generateChangeLog task (issue 2), so that the plugin fails if the changelog file already exists.

2) Removed try/catch blocks from Main.java (issue 3).  Now when a change set runs into a problem, we get a build failure and short message about what went wrong instead of a double stack trace and a success.

3) I took a stab at a README.md file because the change from Conventions to Extensions requires a change in configuration for users' build.gradle files.  It's pretty rough, but I think it at least conveys the changes that need to be made.  I'm sure the README.md can be greatly improved.

4) Added the idea plugin to build.gradle for users that want to check out the code and look at it in Idea.

Thank you for providing us with a great plugin.  It has saved us a lot of headaches where I work, where it is now part of our standard build process.

Steve